Fibronectin is a large glycoprotein, about 450 thousand daltons, which is composed of several apparently independent functional domains. Fibronectin was earlier discovered as a major extracellular matrix protein, and it was demonstrated that it would interact in vitro with other structural molecules, such as collagen, glycosaminoglycans, proteoglycans, fibrinogen, fibrin, and actin, as well as with cell surfaces. It was discovered that fibronectin promotes the attachment of suspended cells to collagen and also that it promotes the attachment of suspended cells directly to tissue culture substrate, independent of its binding to collagen. Accordingly, investigation continued with respect to the region of the fibronectin molecule that interacts with cell surfaces.